You can use this mysql query code:
ALTER TABLE yourtable MODIFY
yourcol VARCHAR(255)
CHARACTER SET utf8
COLLATE utf8_general_ci;
There are more problems with ENUM and SET type fields.
For ENUM you have to specify all the field values, no matter how many they are
ALTER TABLE yourtable MODIFY
yourcol ENUM('val1','val2',etc)
CHARACTER SET utf8
COLLATE utf8_general_ci;
For SET the problems rise from the name SET, so you have to do this command
ALTER TABLE yourtable MODIFY
yourcol CHAR(255)
CHARACTER SET utf8
COLLATE utf8_general_ci;
You will be able afterwards to modify CHAR to SET (with your previous values) without losing your data.